1. Field of Invention
The invention relates to new metallocenes and their use as catalysts in olefin polymerization.
2. Prior Art
Metallocenes of the metals of transition group IV of the Periodic Table of the Elements are highly active catalysts for the polymerization of olefins. The resulting polyolefins have new property combinations and supplement the product range of the polyolefins prepared hitherto using known conventional Ziegler-Natta catalysts.
It is known that catalysts based on unbridged, substituted and unsubstituted biscyclopentadienyl metallocenes in combination with aluminoxanes as cocatalyst can be used for the preparation of polyethylene and ethyleneolefin copolymers (EXXON EPA 128 046).
It is also known that stereoregular polyolefins can be prepared using bridged, chiral metallocenes. For bridging the ligand systems, use is mostly made of dimethylsilanediyl groups (CHISSO EPA 316 155), methylphenylsilanediyl groups (HOECHST EPA 320 762), ethylene groups (Brintzinger et al., J. Organomet. Chem., 288 (1985) 63-67) and isopropylidene bridges (Mitsui Toatsu EPA 459 264). Depending on the ligand type and the substituents, isotactic, syndiotactic, hemiisotactic, stereoblock-type and atactic homopolymers and copolymers having aliphatic or cyclic structures can be prepared.
As ligands, preference is given to using substituted and unsubstituted cyclopentadienyl units (CHISSO EPA 316 155), substituted and unsubstituted indenyl units (Hoechst EPA 302 424; Hoechst EPA 485 823) and also substituted and unsubstituted cyclopentadienyl units in combination with unsubstituted fluorenyl groups (Mitsui Toatsu EPA 412 416).
Likewise, it is known that bridged metallocenes having a cyclopentadienyl system and a heteroatom ligand (constrained geometry catalyst) can also be used for the polymerization of olefins (EXXON U.S. Pat. No. 5,096,867).
Among these various types of metallocene, the bridged, chiral, substituted bisindenyl systems have attained particular importance. Thus, it was able to be shown that the type of substituents and the position of the substituents on the ligand of the metallocene have a significant influence on the reactivity of the catalyst system and the stereoregular structure of the polyolefins obtained. Two possible substitution patterns in particular have been found to be advantageous. The first possibility entails substitution of the indenyl ligand in the 2, 4 and/or 6 position (Hoechst EPA 485823; Angew. Chem., 10 (1992) 1373) while the second possibility is fusion onto the benzene ring of the indenyl ligand (Organometallics 1994, 13, 964-970). Both types of catalyst can be used for preparing isotactic polypropylene and ethylene-.alpha.-olefin copolymers.
Multiply substituted indenyl ligands can be prepared only at considerable expense. Relatively simple systems having good activity and containing indenyl, 2-methylindenyl or 2-methylbenz e! indenyl ligands give products, in particular polypropylene, having relatively low molar masses which are too low for many applications and represent the lower limit for industrial use.
It is therefore an object of the invention to find further structural variants of bridged metallocenes as catalysts for the polymerization of olefins which give polyolefins, in particular polypropylene, having relatively high molar masses.